1. Field of the Invention.
This invention relates to a time-temperature indicator and, more particularly, to an indicator that is inactive until it is activated by removing an oxygen barrier.
2. Description of the Prior Art.
Several patents have disclosed the use of color-changing indicators to monitor the time-temperature history of perishables. Among these are U.S. Pat. No. 4,189,399, issued Feb. 19, 1980, to Patel; and U.S. Pat. No. 4,212,153, issued July 15, 1980, to Kydonieus et al.
When the perishable to be monitored has a short useful lifetime and/or requires refrigeration, then it is desirable, if not essential, to use an indicator that is inactive until activated. Patel, U.S. Pat. Nos. 4,208,186, issued June 17, 1980, and 4,276,190, issued June 30, 1981, disclosed diacetylenic compositions having an inactive form that is activated by contact with an activating vapor. Activation of a diacetylenic monomer in salt form by conversion to the acid form was disclosed in U.S. Pat. No. 4,373,032, issued Feb. 9, 1983, to Preziosi et al.
U.S. Pat. No. 3,768,976, issued Oct. 30, 1973, to Hu et al., has disclosed a temperature-time integrating indicator that is based on temperature-dependent oxygen diffusion into a package that includes an aqueous solution of a redox dye. The dye is dark in the reduced state and becomes colorless when it is oxidized. A similar indicator, involving a free radical sensitive dye and a peroxide on a carrier, was disclosed in U.S. Pat. No. 3,966,414, issued June 29, 1976, to Khattab et al.
Dreyer and Harries (Final Report, Contract DA-19-129-AMC-112(N), U.S. Army Natick Laboratories, Natick Mass., February 1963) observed thermochromism in triphenylmethane leucosulfite solutions. However, they found that aqueous malachite green solutions became nonthermochromic when heated or irradiated in open vessels (see also R. N. Macnair, J. Org. Chem. 33, 1945 (1968)).